The offshore production of oil at substantial distances from land or in remote areas often makes it impractical to install a pipeline on the sea floor so that the oil produced from the offshore wells can be pumped directly to shore for storage and/or further transportation or processing. Oil produced under such circumstances is more conveniently delivered directly or by short feeder pipelines to a moored dedicated vessel, such as an oil tanker where it can be stored until transhipment. Mooring a vessel offshore for extended periods presents many problems, including anticipated storm conditions at the mooring location. It is desirable for mooring systems to permit the vessel to weathervane so that the vessel will always face into prevailing seas, current and wind. Because of anticipated storm conditions at potential mooring sites, it is generally not suitable to use conventional ship anchors and mooring lines for long term mooring because of the large size which would be required. A more suitable mooring system for an offshore vessel which permits it to weathervane, employs a pivoting assembly built into the vessel to allow for collection of oil/gas while the vessel is moved about or weathervanes (i.e. rotates).
In mooring vessels offshore it is important to keep a very accurate station. The importance of keeping an accurate station with respect to the ocean's floor is obvious when consideration is given to offshore drilling and the requirement that the drilling string, for example should be relatively stationary against ocean caused shear forces at all times. Production of hydrocarbons from subsea wells also requires a stationary point on the production vessel to avoid undue extension of the flexible riser lines.
The change in heading which is often required by weathervaning is also problematical in that it is difficult to accomplish with conventional, multiple anchor mooring systems because anchor chains must be taken in and paid out to accommodate the change in vessel heading.
Because of the difficulties in changing the vessel heading with multiple anchor mooring systems, known mooring systems employ a centrally mounted swivel which extends completely through the ship from the deck through to the bottom of the vessel. The mooring chains extend through the swivel. When a heading change is required the vessel is effectively rotated about the mooring swivel to a desired heading, the swivel being stationary with respect to the mooring anchors and ocean floor. Typical of such mooring systems are U.S. Pat. Nos. 3,191,201; 3,602,175; 3,440,671; 3,552,343; 3,620,181; 3,605,668; 3,590,407; 3,279,404; and 3,774,562.
Onboard mooring systems have traditionally featured a large well extending from the deck through the bottom of the ship. Due to the size of the opening, the structure of the ship must be specifically engineered to accommodate the size of the opening. In attempting to install an onboard turret mooring system into an existing vessel, the size of the well is so large that the structure of each specific vessel must be analyzed prior to construction. The necessity of the extended engineering and design effort required to retrofit an existing tanker with a large central well may create unacceptable costs and delays in the final delivery of the tanker. Extensive engineering and design effort is required since the openings through the deck and the bottom plating must be very large, in the order of thirty to sixty feet in diameter, to properly transmit the mooring loads into the ship's structure. The presence of such a large opening in the hull adversely effects the longitudinal hull strength of the tanker and often requires significant modifications to achieve the proper desired strength required by regulatory agencies. Additionally, the significant delays encountered due to the engineering and design effort required may sometimes result in the tanker for which the design has been created becoming unavailable. In those situations, the entire design effort is wasted since it was especially tailored for a ship of a particular design.
Since a turret design must be developed with a particular ship in mind, one universal design incorporating several compromises may create significant limitations in flexibility to adapt various mooring line diameters and quantities/size of risers unless the standard unit design is overdesigned for the worst conceivable case. Finally, since a turret mooring system must be specially designed for each ship, firm quotations cannot be obtained from shipyards at the early stage of a project due to the uncertainty about the existing ship's hull structure and how the mooring system will effect such a structure.
Accordingly, the apparatus and method of the present invention address the need for a family of several specific designs which are adaptable for any barge, tanker, drill ship, semi-submersible unit or LNG/LPG carrier for the environmental conditions to be encountered. This family of designs could have a wide range of applicability to various size tankers, such as 50,000 DWT to 400,000 DWT, for example and for service in diverse environmental conditions, such as wave heights of thirty to ninety feet, for example and for a multitude of water depths, such as one hundred to three thousand feet, for example. This family of designs would be suitable for numerous types of vessels such as oil storage barges, existing or proposed drill ships and floating process plants, among others.